This invention relates to a method for manufacturing an enhanced heat transfer device consisting of a metal substrate and randomly distributed metal bodies bonded to the substrate.
One type of enchanced heat transferdevice of the metal bonded variety is described in Milton U.S. Pat. No. 3,384,154, in which the metal particles are stacked and integrally bonded together and to a metal substrate to form inter-connected pores of capillary size. This is a multiple layer surface used to enhance the boiling heat transfer coefficients for fluids of either low surface tension or high surface tension. For use with relatively high surface tension fluids or for boiling at subambient pressure, the Milton-type porours boiling surface should have a relatively large effective pore radius and to form same the base metal powder must be composed of relatively large particles. The preferred method for fabricating the Milton surface is by blending a base metal powder component which ultimately forms the porous matrix of the structure and a lower melting brazing component powder. The dry mixture of the two components is simultaneously applied to a binder coated substrate and subsequently heated to a temperature and for a time sufficient to metal bond the matrix or base metal powder to the substrate.
The foregoing technique requires the use of a brazing component whose particle size is quite small relative to the base metal particles. Small braze metal particles are required in order to avoid excessively high localized concentrations of low melting alloy. Excessive local concentrations of this component result in the solution of an undesirable quantity of base metal component or substrate into the local molten brazing alloy. This in turn may result in collapse of the base metal matrix component causing serious detrimental alteration of the effective pore radius. In addition, the substrate may be weakened or penetrated by solution of an undesirable amount of substrate metal into the molten alloy at locations of high concentrations of braze metal component. Finally, the above described technique for producing the molten surface employing a dry mixture of powders having widely different particle sizes presents a severe problem of maintaining a uniform mixture during the steps involved in applying the mixed powder to the substrate surface. The tendency for the base metal and braze metal components to segregate during normal handling and application of the mixture results in improper distribution of the base metal or matrix component, inadequate bonding between individual particles and with the substrate surface, and in the above described metallurgical problems associated with high localized concentrations of braze metal component.
Another type of enhanced heat transfer device with metal bodies bonded to a metal substrate is described and claimed in a patent application Ser. No. 721,862 entitled "Enhanced Condensation Heat Transfer Device and Method" filed on even date herewith in the name of Frank Notaro. The Notaro-type metal body surface comprises a single layer of randomly distributed metal bodies each individually bonded to the substrate, spaced from each other and substantially surrounded by said substrate. This device is useful, for example, in the enhancement of condensation heat transfer and may be formed from base metal powder which passes through a 30 mesh size U.S. Standard Screen and is retained on a 40 U.S. Standard Screen. The fabrication of this type surface presents problems similar to those enumerated above for the Milton-type surface. Again, a wide disparity in particle sizes is encountered between the base metal powder and the braze metal powder.
An object of this invention is to provide a method for manufacturing an enhanced heat transfer device consisting of a metal substrate and randomly distributed metal bodies bonded to the substrate, employing a braze metal powder of major dimensions substantially smaller than the base metal powder, without reducing the base metal major dimensions.
Another object is such a manufacturing method which does not weaken the substrate metal by excessive solution into the molten alloy.
Still another object is such a manufacturing method which maintains a uniform mixture of base metalbraze metal powder during the steps of applying the mixture to the metal substrate.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claim.